In the art of hydraulic control systems for control of engines, the trend is toward control over more mechanical variables in the engine to attempt an increase in engine efficiency and/or performance. Mechanical variables can include air and fuel valves, variable stator vanes, engine variable geometry, and the like. In prior engines, the common approach of controlling these mechanical variables has been to provide a dedicated hydraulic control for each mechanical variable. However, with the increased number of hydraulic controls has come undesirable increases in weight and size of the overall engine and a decrease in reliability. Such increases in weight and size also decrease the fuel efficiency of engines, particularly for gas turbine aircraft engines.
The concept of multiplexing a single hydraulic or pneumatic control to a plurality of channels is known as exemplified by Leeson et al., U.S. Pat. No. 4,984,505, the disclosure of which is hereby incorporated by reference. Multiplexed systems eliminate or reduce the need for several separate hydraulic controls while increasing overall reliability. Leeson illustrates such a multiplexing configuration in which a selectively positioned modulating valve moves linearly with respect to a rotating and multiplexing sleeve. The multiplexing sleeve periodically or sequentially delivers a modulated flow to individual output ports. In other multiplexing schemes, the multiplexer comprises a linearly moving valve as exemplified by McLevige et al., U.S. Pat. No. 5,048,394, the disclosure of which is also hereby incorporated by reference. In both rotary and linear multiplexing configurations, an intermediate second stage valve may be interposed between each multiplexer output and each actuator to integrate and/or amplify the signal to the actuator.
While these multiplexing systems reduce the number of hydraulic controls and increase reliability, a drawback with these prior hydraulic multiplexing configurations is that the modulating valve and multiplexer are frequently modulating flow to the second stage valves to correct for error and/or to maintain the last position of the intermediate second stage valves. Such frequent modulated flow may be necessary, for example, to correct for gradual fluid seepage from the control chamber of the second stage valve, which can cause the second stage to fall out of the desired position. These frequent modulations may cause fatigue and wear on the components of the system which may in turn reduce the life-span of the system. Such frequent modulations also can require a large quantity of electrical power.
There are also known attempts to configure a multiplexing scheme with latching valves that do not need updating to hold the last valve position. Such a configuration is exemplified in Veilleux, Jr. et al., U.S. Pat. No. 5,551,478. In Veilleux, a plurality of latching second stage valves switch between two positions by application of high pressure signals to one of two control ports corresponding with the two valve positions to change fluid flow to a corresponding actuator. A high pressure pulse on one port switches the valve from a first to a second position and the application of a high pressure pulse to the second port switches the valve back to its first position. The latching valves use internal ports and switches to low and high pressure inputs and an internal spring biasing mechanism to latch the valves in the current position until the appropriate high pressure pulse is delivered to the appropriate control port. However, a problem with this prior latching valve multiplexing system is its size, weight, and complexity, which are a disadvantage in aircraft systems and other systems where smaller size and weight is highly desired. In particular, Veilleux requires a 4-way multiplexing valve that has two control ports for each second stage latching valve. Each latching valve likewise has two control chambers and ports connected by separate conduits to the multiplexing valve. Furthermore, each latching valve requires two high pressure inputs and two low pressure inputs to maintain the latched position and produce an output to an actuator. The numerous ports increase the number of connecting conduits, the overall length or size of the multiplexing and latching valves, and therefore the complexity and weight of the system. Yet another problem with Veilleux is that the disclosed multiplexed fluid control system only provides positive high pressure pulses, and therefore it is not compatible with other variably positioned second stage valves which operate on positive and negative fluid signals. Such variably positioned second stage valves offer better control over mechanical variables which prefer more accurate control.